If you do any amount of riding you will encounter rain and will appreciate how a bicycle helmet cover can help keep your head nice and dry. And they also have the added benefit of providing an extra layer covering the helmet vents to help keep out the wind and your head warmer if riding in colder weather.
Many styles are bright yellow providing extra visibility for motorists while some also feature reflective elements for the same purpose. I find that when I am bicycle touring and must get to my next destination and cannot avoid the rain that these are useful. Bike commuters will love this bike helmet covers for the same reason, check them out below.